U.S. patent number 7,301,560 [Application Number 10/853,741] was granted by the patent office on 2007-11-27 for three-dimensional field for calibration and method of photographing the same.
This patent grant is currently assigned to Topcon Corporation. Invention is credited to Tadayuki Ito, Nobuo Kochi, Takayuki Noma, Hitoshi Otani, Mitsuharu Yamada.
United States Patent |
7,301,560 |
Noma , et al. |
November 27, 2007 |
Three-dimensional field for calibration and method of photographing
the same
Abstract
A three-dimensional field for calibration having a wide-angle
area 110 and a zooming area 120 located within an area overlapped
with the wide-angle area 110 comprises: a plurality of rough
alignment reference marks 122 for zooming and a plurality of
precise alignment reference marks 124 for zooming, and the rough
alignment reference marks 122 and the precise alignment reference
marks 124 for zooming being arranged within the zooming area 120;
and a plurality of rough alignment reference marks 112 for
wide-angle and a plurality of precise alignment reference marks 114
for wide-angle, the rough alignment reference marks 112 and the
precise alignment reference 124 marks for wide-angle being arranged
within the wide-angle area 110.
Inventors: |
Noma; Takayuki (Tokyo,
JP), Ito; Tadayuki (Tokyo, JP), Otani;
Hitoshi (Tokyo, JP), Yamada; Mitsuharu (Tokyo,
JP), Kochi; Nobuo (Tokyo, JP) |
Assignee: |
Topcon Corporation (Tokyo,
JP)
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Family
ID: |
33128190 |
Appl.
No.: |
10/853,741 |
Filed: |
May 26, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050012844 A1 |
Jan 20, 2005 |
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Foreign Application Priority Data
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May 26, 2003 [JP] |
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2003-147209 |
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Current U.S.
Class: |
348/187 |
Current CPC
Class: |
G01C
11/00 (20130101); G01C 25/00 (20130101) |
Current International
Class: |
H04N
17/00 (20060101) |
Field of
Search: |
;348/187,188,189,175,177,180,181,184,185 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1 378 790 |
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Jan 2004 |
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EP |
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1 484 576 |
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Dec 2004 |
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EP |
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2001-280956 |
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Oct 2001 |
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JP |
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Primary Examiner: Natnael; Paulos M.
Attorney, Agent or Firm: Foley & Lardner LLP
Claims
What is claimed is:
1. A three-dimensional field for calibration having a wide-angle
area and a zooming area located within an area overlapped with the
wide-angle area, comprising: a plurality of rough alignment
reference marks for zooming and a plurality of precise alignment
reference marks for zooming, the rough alignment reference marks
and the precise alignment reference marks for zooming being
arranged within the zooming area; and a plurality of rough
alignment reference marks for wide-angle and a plurality of precise
alignment reference marks for wide-angle, the rough alignment
reference marks and the precise alignment reference marks for
wide-angle being arranged within the wide-angle area.
2. The three-dimensional field for calibration of claim 1, wherein
each of the reference marks is constituted of a reflective
target.
3. The three-dimensional field for calibration of claim 1, wherein
at least one of the rough alignment reference marks for wide-angle
is generally positioned at the center of the wide-angle area and
within the zooming area; the precise reference marks for zooming
are positioned within the zooming area; and at least one of the
precise alignment reference marks for wide-angle is positioned
within the wide-angle area surrounding the zooming area.
4. The three-dimensional field for calibration of claim 1, wherein
the rough alignment reference marks for zooming or wide-angle are
formed in a distinct mode in which they are distinct from the
precise alignment reference marks for zooming or wide-angle.
5. The three-dimensional field for calibration of claim 1, wherein
at least one among the rough alignment reference marks for zooming
or wide-angle and the precise alignment reference marks for zooming
or wide-angle has a height different from a height of other
reference marks.
6. The three-dimensional field for calibration of claim 1, wherein
the rough alignment reference marks for wide-angle have a geometry
larger than that of the rough alignment reference marks for
zooming; and the precise alignment reference marks for wide-angle
have a geometry larger than that of the precise alignment reference
marks for zooming.
7. The three-dimensional field for calibration of claim 1, wherein
at least one among the rough alignment reference marks for
wide-angle and the rough alignment reference marks for zooming has
a geometry to be used as both the rough alignment reference marks
for wide-angle and the rough alignment reference marks for
zooming.
8. A method of photographing with a camera for photographing a
three-dimensional field for calibration of claim 1, to obtain an
image of the calibration, comprising the steps of: setting the
exposure of the camera for photographing a calibration image to be
overexposed for an background, excluding the reference marks of the
three-dimensional field for calibration, and to be underexposed for
the reference marks of the three-dimensional field for calibration;
photographing the three-dimensional field for calibration by
strobing with the camera for photographing to obtain a calibration
image; and forming the image of the calibration so as to highlight
the contrast thereof.
Description
BACKGROUND OF TH INVENTION
This invention relates to a three-dimensional field for calibration
for use in exactly measuring internal parameters, such as a
principal point position, a screen distance (focal length) and a
distortion parameter of a lens, necessary to correct an image
distortion of a camera. This invention also relates to a method of
photographing a three-dimensional field for calibration to obtain a
calibration image for use in measuring internal parameters of a
camera.
Conventionally, in the field of photogrammetry or photographic
measurement it has been important to obtain an image with less
aberration. For this purpose, in these fields a high-accuracy lens
with small aberration has been used for a photographic camera, as
disclosed in JP-A-2001-280956. Further, in the field of
phtogrammetry, each of the multiple points positioned and precisely
measured in three dimension is measured in plural directions and
then the internal parameters, such as a principal point position, a
screen distance (focal length) and a distortion parameter, of a
camera are analytically obtained, while, in the field of
photographic measurement, the internal parameters of a fabricated
camera are obtained by precisely measuring the camera.
Recently a digital camera with an image pick up device (charge
coupled image sensor) is in widespread use. There are provided a
number of models of the digital camera having a so called zoom lens
capable of varying a focal length thereof. Use of a zoom lens is
effective to adjust an area or size that the image of an object to
be measured occupies in an image projected on a film because it is
unnecessary to move the camera relative to the object to be
measured. Accordingly, it will become easy to perform a
three-dimensional image measurement if the zoom lens can be applied
to the photographic measurement. However, there is a problem in use
of the zoom lens that a calibration work becomes complicated due to
a variation of internal parameters of the camera depending on a
change in the focal length of the zoom lens.
Specifically, when a three-dimensional field for calibration having
multiplicity of points (targets) that are precisely measured and
positioned thereon in three dimensions is taken with a camera to be
calibrated, it is preferable that the three-dimensional field for
calibration is taken so as to occupy fully an image projected on a
film or CCD. Unfortunately, the size of an object to be
photographed varies according to the variation in focal length of
the zoom lens. For photographing the three-dimensional field for
calibration so as to fully occupy the projected image, it is
therefore necessary to shorten the distance between the
three-dimensional field for calibration and camera under a
wide-angle condition, while it is necessary to lengthen it under a
zooming condition. Under some zooming conditions, the required
distance between the three-dimensional field for calibration and
camera is, for example, about 10 to 20 meters, which requires an
expansive space for the three-dimensional field for calibration to
be placed. It is therefore a further problem that a space available
for photographing the three-dimensional field for calibration and
performing an associated calibration work calibrate is limited.
In photographing the three-dimensional field for calibration, it is
also preferable that the targets located in the three-dimensional
field for calibration are derived in a mode in which those targets
can be clearly recognized and taken with a camera to be calibrated.
On the other hand, in many cases, near the three-dimensional field
for calibration are placed tools or instruments regardless of
calibration and they may be projected as inutile objects on an
photographed image of the three-dimensional field for calibration.
Accordingly, it is also a problem that there is provided a noise or
bar for a calibration work performed every focal length of a zoom
lens.
SUMMARY OF THE INVENTION
The invention was made to solve the problems described above and a
first object of the invention is to provide a three-dimensional
field which allows to photograph a three-dimensional field for
calibration and calibrate using a zoom lens capable of varying its
focal length even though in a relatively small space. A second
object of the invention is to provide a method of photographing a
three-dimensional field for calibration which allows targets in a
three-dimensional field for calibration to be photographed in a
mode in which they can be clearly recognized.
To realize those objects, the three-dimensional field having, as
shown in FIGS. 1 and 2, a wide-angle area 110 and a zooming area
120; located within an area overlapped with the wide-angle area
110, comprises: a plurality of rough alignment reference marks
(targets) 122 for zooming and a plurality of precise alignment
reference marks (targets) 124 for zooming, the rough alignment
reference marks 122 and the precise alignment reference marks 124
for zooming being arranged within the zooming area 120; and a
plurality of rough alignment reference marks (targets) 112 for
wide-angle and a plurality of precise alignment reference marks
(targets) 114 for wide-angle, the rough alignment reference marks
112 and the precise alignment reference marks 114 for wide-angle
being arranged within the wide-angle area 110.
In such an apparatus, the wide-angle area 110 is used to photograph
for obtaining a calibration image on the wide-angle side of a zoom
lens and also provided with the rough alignment reference marks 112
for wide-angle for use in a rough alignment in a photographed
calibration image and with the precise alignment reference marks
114 for wide-angle for use in a precise alignment in the
photographed calibration image. The zooming area 120 is used to
photograph for obtaining a calibration image on the zooming side of
the zoom lens and also provided with the rough alignment reference
marks 122 for zooming for use in a rough alignment in a
photographed calibration image and with the precise alignment
reference marks 124 for zooming for use in a precise alignment in a
photographed calibration image. Since the zooming area 120 is
located in a small area overlapped with the wide-angle area 110,
the calibration image can be taken even if it is difficult to
satisfactorily secure a distance between the photographic camera
and the three-dimensional field.
In the inventive three-dimensional field for calibration, it is
preferred that each of the reference marks (112, 114, 122, 124) is
constituted of a reflective target. Therefore, the reference marks
in the calibration image taken with the camera have a high
intensity and the contrast between the reference marks and the
background surface of the three-dimensional field for calibration
can be easily secured.
In the inventive three-dimensional field for calibration, it is
preferred that at least one of the rough alignment reference marks
112 for wide-angle is generally positioned at the center of the
wide-angle area 110 and within the zooming area 120, the precise
reference marks 124 for zooming are positioned within the zooming
area 120, and at least one of the precise alignment reference marks
114 for wide-angle is positioned within the wide-angle area 110
surrounding the zooming area 120. The consistency between the
wide-angle area 110 and the zooming area 120 is increased and both
the wide-angle area 110 and the zooming area 120 are available for
calibration at intermediate focal lengths corresponding to the
boundary area between the wide-angle area 110 and zooming area
120.
In the inventive three-dimensional field for calibration, it is
preferred that the rough alignment reference marks (112, 122) for
zooming or wide-angle are formed in a mode in which they are
distinct from the precise alignment reference marks (114, 124) for
zooming or wide-angle. It is therefore easy to distinguish between
the reference marks for rough alignment and the reference marks for
precise alignment. This mode means a mode allowing to be distinct
with at least one of a form, color and pattern, for example.
In the inventive three-dimensional field for calibration, it is
preferred that at least one among the rough alignment reference
marks (112, 122) for zooming or wide-angle and the precise
alignment reference marks (122, 124) for zooming or wide-angle has
a height different from a height of other reference marks. It is
therefore possible to calibrate three dimensionally in a
calibration image taken with a camera.
In the inventive three-dimensional field for calibration, it is
preferred that the rough alignment reference marks 112 for
wide-angle have a geometry larger than that of the rough alignment
reference marks 122 for zooming; and the precise alignment
reference marks 114 for wide-angle have a geometry larger than that
of the precise alignment reference 124 marks for zooming.
Accordingly, the reference marks for zooming which were
photographed on the wide-angle side have a small size in a
photographed calibration image. It is therefore easy to identify
the reference marks for wide-angle. Also, the reference marks for
zooming which were photographed on the zooming side are projected
with a size suitable to be identified.
In the inventive three-dimensional field for calibration, it is
preferred that at least one among the rough alignment reference
marks 112 for wide-angle and the rough alignment reference marks
122 for zooming has a geometry to be used as both the rough
alignment reference marks (112, 122) for wide-angle and zooming.
The consistency between the wide-angle area 110 and the zooming
area 120 is increased and both the wide-angle area 110 and the
zooming area 120 are available for calibration at intermediate
focal lengths corresponding to the boundary area between the
wide-angle area 110 and zooming area 120.
To realize those objects, the inventive method of photographing a
three-dimensional field for calibration according to the invention
is, as shown in FIG. 5, a method of photographing with a camera for
photographing a three-dimensional field for calibration, of any one
of claim 1 to claim 7, to obtain an image of the calibration,
comprising the steps of: setting the exposure of the camera for
photographing a calibration image to be overexposed for an
background, excluding the reference marks of the three-dimensional
field 100 for calibration, and to be underexposed for the reference
marks of the three-dimensional field 100 for calibration (S100);
photographing the three-dimensional field (100) for calibration by
strobing with the camera for photographing a calibration image
(S102); and forming the image of the calibration so as to highlight
the contrast thereof (S104).
This application is based on Japanese patent application, No.
2003-147209 filed in Japan on May 26, which is entirely
incorporated herein by reference.
The present invention will become more fully understood from the
detailed description given hereinbelow. However, the detailed
description and the specific embodiment are illustrated of desired
embodiments of the present invention and are described only for the
purpose of explanation. Various changes and modifications will be
apparent to those ordinary skilled in the art on the basis of the
detailed description.
The applicant has no intention to give to public any disclosed
embodiment. Among the disclosed changes and modifications, those
which may not literally fall within the scope of the patent claims
constitute, therefore, a part of the present invention in the sense
of doctrine of equivalents.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a overall view of a first embodiment of the invention,
illustrating with a light and shade binary image;
FIG. 2 is a partially enlarged view of FIG. 1 as a overall view,
illustrating a zooming area with a light and shade binary
image;
FIG. 3 is a overall view, illustrating a site where a
three-dimensional field 100 for calibration is placed;
FIG. 4 is a partially enlarged view of FIG. 3 as a overall view,
illustrating a zooming area; and
FIG. 5 is a flow chart, illustrating a procedure of photographing a
three-dimensional field for calibration with a zoom lens to be
calibrated.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is hereinafter described with reference to the
accompanying drawings, in which: FIG. 1 is a overall view of a
first embodiment of the invention, illustrating with a light and
shade binary image; and FIG. 2 is a partially enlarged view of FIG.
1 as a overall view, illustrating a zooming area with a light and
shade binary image. A three-dimensional field 100 has a wide-angle
area 110 and a zooming area 120 which is located within an area
overlapped with the wide-angle area 110.
The wide-angle area 110 is used to take a calibration image on the
wide-angle side of a zoom lens and also provided with a rough
alignment reference marks (targets) 112 for wide-angle for use in
rough alignment in a photographed calibration image and provided
with precise alignment reference marks (targets) 114 for wide-angle
for use in precise alignment in the photographed calibration image.
In the vicinity of the zooming area 120 are provided seven rough
alignment reference marks 112 for wide-angle so that it is secured
to photograph even though the zoom lens has a smaller angle of
view. For example, 100 to 200 of the precise alignment reference
marks 114 for wide-angle are evenly distributed with a density so
as to be projected evenly on a photographed calibration image. The
rough alignment reference marks 112 for wide-angle have a center in
a shape of, for example, a circle (a star and a triangle also
permitted) and a closing line in a shape of a rectangular which
circumscribes the center. Alternatively, the closing line may be in
a shape of a circle and a polygon other than a triangle. In other
words, any other shape by which the rough alignment reference mark
112 for wide-angle can be clearly identified may be used.
The zooming area 120 is used to take a calibration image on the
zooming side of the zoom lens and also provided with rough
alignment reference marks (targets) 122 for zooming for use in
rough alignment in a photographed calibration image and provided
with precise alignment reference marks (targets) 124 for zooming
for use in precise alignment in the photographed calibration image.
Since the zooming area 120 is located in a small area surrounded by
the wide-angle area 110, the calibration image can be photographed
with a zoom lens which has a small angle of view and of which focal
length is set on the zooming side, even if an distance between the
photographic camera and the three-dimensional field is not
enough.
In the vicinity of the center of the zooming area 120 are provided
seven rough alignment reference marks 122 for zooming so that it is
secured to photograph even though the zoom lens has a smaller angle
of view. For example, 100 to 200 of the precise alignment reference
marks 124 for zooming are evenly distributed with a density so as
to be projected evenly on the photographed calibration image. The
rough alignment reference marks 122 for zooming have a center and a
closing line, each in the same shape as of the rough alignment
reference marks 124 for zooming. Alternatively, the closing line
may be in a shape of a circle and a polygon other than a triangle.
In other words, any other shape by which the rough alignment
reference mark 122 for zooming can be clearly identified may be
used.
The rough alignment reference marks 112 for wide-angle have a
geometry larger than that of the rough alignment reference marks
122 for zooming; and the precise alignment reference marks 114 for
wide-angle have a geometry larger than that of the precise
alignment reference 124 marks for zooming. Accordingly, the
reference marks 122, 124 for zooming which were photographed on the
wide-angle side of the zoom lens have a small size in the
photographed calibration image. It is therefore easy to identify
the reference marks 112, 114 for wide-angle. Also, the reference
marks 122, 124 for zooming which were photographed on the zooming
side of the zoom lens are projected with a size suitable to be
identified in the photographed calibration image.
Next, a site where the three-dimensional field 100 for calibration
is placed will be described, with reference to the drawings. FIG. 3
is an overall view, illustrating the site where the
three-dimensional field 100 for calibration is placed, and FIG. 4
is a partially enlarged view of FIG. 3 as an overall view,
illustrating a zooming area. The three-dimensional field 100 is
provided in an environment of a less variation of temperature, for
example, a basement or cellar so that the relative positions
between the reference marks for wide-angle and the reference marks
for zooming cannot be varied. On a wall surface, such as a concrete
wall, is fixed a target board indicating the reference marks 112,
114 for wide-angle and the reference marks 122, 124 for
zooming.
For each of the reference marks 112, 114, 122, 124, a reflective
reference target having a high reflectivity is used, resulting in
an easy photographing of a calibration image as a light and shade
binary image. By use of a reference mark having a high
reflectivity, such as a glass plate, on which aluminium is
deposited, the reference marks having a high intensity are
projected on a photographed calibration image. It is therefore easy
to obtain a sufficient contrast between the reference marks and the
background of the three-dimensional field for calibration. The
height of the reference mark is adjusted by changing the height of
the target board from the surface of a concrete wall, for example.
Evenly locating the reference marks having a variety of heights all
over the calibration image, internal parameters of the zoom lens to
be calibrated are exactly calculated.
FIG. 5 is a flow chart, illustrating a procedure of photographing a
three-dimensional field for calibration with a zoom lens to be
calibrated. First of all, the exposure of the camera for
photographing a calibration image is set to be overexposed for an
background excluding the reference marks of the three-dimensional
field for calibration, and to be underexposed for the reference
marks of the three-dimensional field for calibration (S100).
Secondly, the three-dimensional field for calibration is
photographed by strobing with the camera for photographing a
calibration image (S102). In this step, since the calibration image
can be photographed with the zoom lens for which aperture is closed
at a minimum value, a depth of field becomes deep. It is therefore
easy to obtain a clear image for analysis even if be photographed
out of focus.
Subsequently, the image of the calibration is formed so as to
highlight the contrast thereof (S104). The image of the calibration
photographed in such a manner is a binary image in which a
background image, such as a concrete wall surface, of the
three-dimensional field for calibration is shaded and both the
reference marks 112, 114 for wide-angle and the reference marks
122, 124 are lightened (S106).
The image of the calibration obtained in a way as described, is
used for comparatively calculating the measured three-dimensional
position of the formed binary image and the three-dimensional
coordinates of the reference marks 112, 114 for wide-angle and the
reference marks 122, 124 for zooming to analytically obtain
internal parameters (a principal point position, screen distance
and distortion parameter) of the camera every focal length of the
zoom lens. The background area, excluding the reference marks of
the three-dimensional field 100 for calibration, is brought to an
evenly shaded area of the image. Accordingly, in a calibration work
as a post-process using the image of the calibration, both the
reference marks 112, 122 for wide-angle and the reference marks
122, 124 for zooming can be exactly detected. For example, the
experiment performed by the inventors shows that the detection
accuracy of a median point of the reference mark (target) is
approximately 1/10 in the binary image, while that in the image on
which a wall surface in the background was projected is
approximately 1/4, that is, degraded about two to three times of
the former.
Although, in the embodiment described above, both the reference
marks for wide-angle and the reference marks for zooming are
commonly fixed on the wall surface, the invention is not limited to
it. For example, the reference marks for wide-angle are fixed to a
wall surface, the reference marks for zooming may be fixed to a
movable panel so that the reference marks for zooming is moved and
positioned relative to the wall surface for positioning the
reference marks for wide-angle and the reference marks for
zooming.
As is described above, with use of the three-dimensional
calibration field of the invention, a photographed calibration
image can be obtained, without adjustment of the distance between
the photographic camera and the three-dimensional field for
calibration, by photographing the wide-angle area when the focal
length exists on the wide-angle side and by photographing the
zooming area when on the zooming side, even if a sufficient
distance between the camera and the three-dimensional field for
calibration cannot be securely obtained.
Also, according to the method of photographing the
three-dimensional field for calibration of the invention, since the
calibration image can be photographed with the zoom lens for which
aperture is closed at a minimum value, a depth of field becomes
deep. It is therefore easy to obtain a clear image for analysis
even if be photographed out of focus. When the calibration image is
photographed so as to obtain the calibration image as a binary
image, a higher detection accuracy of median point of each
reference mark (target) can be provided in the calibration
work.
* * * * *